"We have taken the first picture of a black hole! This is an extraordinary scientific feat accomplished by a team of more than 200 researchers." with these words, the Event Horizon Telescope (EHT) project director Sheperd S. Doeleman of the Center for Astrophysics | Harvard & Smithsonian presented the first ever image of a black hole earlier this Spring. EHT links telescopes around the globe to form an Earth-sized virtual telescope with unprecedented sensitivity and resolution. It took two decades of work to capture the image. Part of that effort was designing, building, and hauling the hardware to various telescope sites to a synchronization level of a billionth of a second. But they also had to anticipate what they might see by nailing down the physics of black holes as accurately as possible. It is the result of years of international collaboration, and offers scientists a new way to study the most extreme objects in the Universe predicted by Einstein’s general relativity during the centennial year of the historic experiment that first confirmed the theory. In this issue of the EP newsletter we interview Sheperd S. Doeleman about the steps that led to this success, the future plans and what these observations could reveal about gravity.

When the idea of "photographic" a black hole was originally conceived? Does this coincide with the birth of the EHT?

Hilbert (1916), van Laue (1921) and others described the 'impact parameter' of a black hole very early on: the apparent size of the photon orbit we would see at infinity if a black hole were illuminated from behind. Interior  to the photon orbit, light paths intersect the event horizon and cause a dark region. The EHT is a cutting edge experiment, but with roots that go back 100 years!

After the full Kerr metric for spinning black holes was described (1963), Bardeen worked out the size and shape of this 'silhouette' or 'shadow' feature in the general case.  Jean-Pierre Luminet (1979) then made the first full simulations of what a black hole would look like if it had a thin accretion disk, and Falcke, Agol & Melia (2000) described the case for an optically thin accretion flow.  In parallel, observers were refining the technique of Very Long Baseline Interferometery (VLBI) which links telescopes across the globe to give an effective telescope size that is an Earth diameter.

These developments, both from the theory and observational side, converged in the 2000's.  My group took on the challenge of imaging a black hole by re-inventing the VLBI instrumentation to greatly expand the bandwidth, thereby achieving the sensitivity required to detect both SgrA* and M87 (the two supermassive black holes with the largest apparent silhouettes or shadows) on long baselines that resolve the photon orbit. Finally in 2007 we made  observations of SgrA* that showed conclusively that there was structure measuring only ~4 Schwarzschild radii across (Doeleman et al. 2008), and in 2009 we formed the EHT project (Doeleman et al. 2009), with detections of horizon-scale structure made for M87 in 2009 (Doeleman et al. 2012).

These first detections were the needed breakthrough that allowed us to grow an international team and build out the EHT to an imaging array.  The black hole photon orbits of SgrA* and M87 are about 40-50 micro arcseconds on the sky, so we needed to develop techniques to resolve these small structures.

How this photograph was taken?

We use Very Long Baseline Interferometry (VLBI) where radio waves from the black hole are collected by dishes across the globe and recorded with precision timing provided by atomic clocks.

After the observations, in which all the dishes track the same black hole at exactly the same time, the recordings are shipped to a central facility - nothing beats the bandwidth of a plane filled with hard disk drives!-  where they are combined, much as an optical mirror combines all the light that hits it at the focus. The result is a virtual telescope, or perhaps more accurately interferometer, as big as the distance between the observing dishes.

The resolution of this telescope is given by the wavelength of light divided by the distance between the telescopes. The angular resolution of the EHT, which comprises 8-10 dishes, is sufficient to resolve the black hole shadows of our two main targets.

The supermassive black hole at the core of supergiant elliptical galaxy Messier 87, with a mass ~7 billion times the Sun's, as depicted in the first image released by the Event Horizon Telescope. The dark circle in the middle of the image is the "shadow" of the black hole as revealed by the overheated glowing gas that sits at M87's event horizon. The extreme gravitational pull of the black hole heats the gas that emits a form of radiation or "glow". The EHT captured radiowaves with a wavelength of 1.3 mm, emitted by the swirling gas. The gas emits light at different wavelengths but this particular wavelength can travel through entire galaxies. The yellow tones in the image represent the most intense emissions, while red depicts lower intensity and black represents little or no emissions. The light that we see in this image isn’t just coming from the sides of the black hole but from all directions. Space and time are so warped, that some of the light orbits the black hole in a circle.

What can we learn about black holes by direct observation?

By fitting the size and shape of the photon orbit, we can test General Relativity predictions.  We have done this for M87, but because the distance to M87 is uncertain at the 10-20% level, this limits our GR tests.  However, if we can time the period of orbits of matter circling the BH, then that would be a separate test of GR. We have also been able to weigh the M87 black hole to within the precision of the distance to be 6.5 billion solar masses.  Since this mass is within the photon orbit, it is the best evidence for the existence of super massive black holes to date. Last, even for scientists, 'seeing is believing', so finally being able to see a black hole and the shadow that is caused by light lost into the event horizon, is a tremendous moment, not just for the EHT team, but for everyone.

Should we expect a second photograph soon?

One should note that we can't see stellar mass black holes with the EHT as they are too small.  But we are currently working on SgrA*, which is 4 million solar masses. I am afraid though that we can't yet announce when those results will be released.

Is there some kind of complementarity in the information we can/could get by looking with VLBI and with gravitational waves detectors?

LIGO and EHT are complementary. With LIGO  we observe cataclysmic events that cannot be studied again and this allows to study General Relativity when spacetime metrics are changing.  EHT can study the same black hole over time including supermassive black holes and more stable GR spacetimes.

Which are the next steps for the Event Horizon Telescope?

We want to make movies! By increasing the number of telescopes, by putting telescopes into space, and by increasing the observing frequency we can move to making sequential snapshots and form real-time movies of black holes in action.   

Can observations with EHT or future similar facilities help us develop a theory of quantum gravity?

It is possible that quantum gravity effects could be observed by the EHT on horizon scales, but it is unlikely.  Still, the only thing we can do is to push forward and make the best measurements we can. The EHT has opened a new window onto black holes and allows us to make precision measurements near the event horizon. We don't want to limit what we might see.

Did you expect this global media attention following the release of the first image?

The media response was tremendous and clearly the EHT results have struck a chord with people on a human level.  I think this is because the result is something that nature seems to tell us is impossible, yet a group of scientists working together across borders and disciplines managed to do it, guided by a strong common science vision.  Building this team has been the accomplishment of a lifetime, and this image could not have been achieved without such a dedicated group of collaborators.

Note: Credits for all image: EHT Collaboration